For electronic devices such as digital camcorders, digital cameras, mobile phones, laptop notebooks, car navigation systems, and headsets, the functions of voice recording or voice communication can be installed thereon. To improve the directivity of the recording quality of an audio signal generated by a signal source from a specific direction, a beamforming technique is developed. The beamforming technique is capable of recording the target audio signal transmitted from a specific direction and removing the unwanted audio signals transmitted from other directions.
FIGS. 1A˜1C are diagrams illustrating the beamforming technique. The audio recording system 1r comprises a signal processing circuit 10 and microphones 12a, 12b. The microphones 12a, 12b are non-directional microphones, which are installed along the directional axis 14 and are separated by a specific distance.
The signal processing circuit 10 receives the audio signals S1a and S1b in the form of electrical signals which are received and converted by the microphones 12a, 12b respectively. The audio signal processing circuit 10 comprises a delay element 11 for delaying the audio signal S1b. For clearly recording the audio signal, the audio signal processing circuit 10 performs the beamforming process to extract the target audio signal from the direction of the center of the directional axis 14. The delay amount T of the delay component 11 is set by the value such that the detecting voltage level corresponding to the audio signal in the opposite direction of the directional axis 14 is substantially zero. Since the beamforming technique is well known to those skilled in the art, the following paragraphs are the brief description of the beamforming technique and the detailed description is omitted for brevity.
FIG. 1A illustrates a circumstance where an audio source 2 has a direction the same as the directional axis 14. FIG. 1B illustrates a circumstance where the audio source 2 has a direction perpendicular to the directional axis 14. FIG. 1B illustrates a circumstance where the audio source 2 has a direction opposite to the directional axis 14. FIGS. 2A˜2C illustrate the waveforms of the audio signals obtained in FIGS. 1A˜1C respectively. For the sake of description, the horizontal and vertical axes of the waveforms or timing diagrams in the specification are adaptively enlarged or reduced. In addition, the waveforms are also being simplified or emphasized for the sake of description.
The two microphones 12a and 12b are only separated by a few centimeters. Therefore, the sound signals 4 generated by the audio source 2 almost have the same amplitude when the sound signals 4 are inputted into the two microphones 12a and 12b, and their phase difference Δφ is varied according to the direction of the audio source 2. As shown in FIG. 1A, when the direction of the audio source 2 is the same as the directional axis 14, the phase difference of the two audio signals S1a and S1b is increased. On the other hand, as shown in FIG. 1B, when the direction 16 of the audio source 2 is perpendicular to the directional axis 14, the phase difference of the two audio signals S1a and S1b is close to zero.
The gain difference (amplitude difference) and/or phase difference of the audio signals S1a and S1b outputted from the microphones 12a and 12b can be used in the beamforming technique. When the two waveforms are the same, the gain difference or the phase difference are essentially equivalent, and the difference (S1a-S1b) of the two audio signals S1a and S1b is correlated. Hence, the audio signal processing circuit 10 can collect the sound signal 4 transmitted from the directional axis 14 by processing the differential signal (S1a-S1b).